Metal oxide semiconductor (MOS) transistors, also known as MOSFETs, have several sources of series electrical resistance, including resistance of metal interconnects to source and drain regions, and resistance of source and drain regions. Accurate measurement of series resistance is important for assessing integrated circuit performance. The voltage—current relationship of MOS transistors is not linear, making extraction of the series resistance problematic. The current art addresses this problem by utilizing multiple transistor test structures, varying parameters such as channel length and width, to improve the accuracy of estimates of series resistance. This is disadvantageous due to increased cost and complexity of transistor testing.
Also, there exists an important need to measure series resistance of MOS transistors in production environments, under constraints of minimization of test time, minimization of test structures, and maximum utilization of test connections. Challenges of production testing of MOS transistor series resistance are increased by a need to perform measurements on wafers using probing equipment, which makes electrical contacts to devices being measured with metal probes. Contact resistances of probe tips to devices being tested are often a significant fraction of total series resistances of transistors, and are frequently variable and unrepeatable from probe contact to probe contact. The current art utilizes Kelvin connections (separate probes for current and voltage for each circuit node tested) to reduce contributions of probe contact resistance to uncertainty in estimates of transistor series resistance. This is disadvantageous due to increased test structure complexity and inefficient use of test connections.
Another need for measuring series resistance of MOS transistors occurs when integrated circuits are electrically analyzed, often after some disassembly or deprocessing. In this situation, transistors which are components of integrated circuits are measured in situ, without benefit of Kelvin connections or multiple test structures. Moreover, the feature sizes of transistors in state of the art integrated circuits are often below one micrometer, which exacerbates the problems of probe contact variability and repeatability.